In-mold labeling is a technique, whereby a label is applied to a molded part during the molding of the part. During a typical in-mold labeling process, a label is inserted into a molding cavity of a molding apparatus, the molding cavity is closed, and molding material is introduced into the closed molding cavity as a melt. When the molding cavity is reopened at an appropriate stage, an in-mold labeled article is ejected from the molding cavity and is decorated with the label. The label itself generally is made of a material having a composition compatible to the material of the molded article.
Successful production of in-mold labeled articles requires that the label be held in a proper location within the mold cavity from when the label is introduced until the molding material solidifies, including the times during which the mold cavity is closed and during which the molding material is introduced into the closed molding cavity. Moreover, to facilitate part ejection and to ensure integrity of the label, the label should not stick to the walls of the molding cavity once the molded part has solidified.
Conventional in-mold labeling apparatus may include mechanical means to hold labels in their proper position. For example, a vacuum system can be integrated into a labeling apparatus such that, through small holes in the mold cavity, a label can be held in place by suction. Such a vacuum system requires capital costs and maintenance costs. Alternatively, an electrostatic generator can be integrated into the labeling apparatus and configured to charge the mold cavity. The charged mold cavity holds the labels in place by electrostatic pinning. Like the vacuum system, the electrostatic generator adds capital costs and maintenance costs. Moreover, the usefulness of electrostatic systems may be limited by charge dissipation during a molding cycle or by incompatibilities with certain specialty labels or metallic labels that cannot hold an electrostatic charge.
Conventional in-mold labeling processes also may involve one or more functional coatings on either the label or the molding apparatus. The functional coatings may include adhesives or antistatic layer, for example and may be incorporated into a molding system as alternatives to, or in addition to, mechanical holding means in the labeling apparatus. For example, the label or the mold-cavity surface may be coated with an appropriate adhesive that holds the label to the mold cavity during molding but debonds from the mold cavity in time for the molded part to be ejected. Methods involving such functional coatings or adhesives, therefore, require a cumbersome and costly step of applying the coating and also require choosing a coating compatible with the label material.
There remain ongoing needs for in-mold labeling systems and in-mold labeling methods having dependable and reproducible features that ensure stabile placement of labels during a molding process.